The amount of carbon dioxide in the tropical Pacific Ocean has increased surprisingly quickly over the past 14 years, according to new research from scientists at the National Oceanic and Atmospheric Administration and the University of Washington.

The reason for the rapid increase in carbon dioxide concentrations is a combination of natural variability and human-caused emissions of carbon dioxide into the atmosphere, said Adrienne Sutton, a research scientist with NOAA's Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington.

Although it is difficult to tease out exactly how much of the increase in carbon dioxide concentrations is due to human-caused climate change, "we assume that most of the carbon dioxide increase [in the tropical Pacific] is due to anthropogenic CO2," Sutton said.

In fact, the rate of increase in carbon dioxide concentrations in parts of the tropical Pacific Ocean, a band of ocean along the equator between the coasts of South America and Southeast Asia, is greater than the rate of increase of atmospheric carbon dioxide.

Measurements by atmospheric scientists at NOAA show that atmospheric CO2 is increasing at a rate of about 2 parts per million per year. But in parts of the tropical Pacific, the rate of change in CO2 concentrations measured by the researchers reached 3.3 ppm per year.

"It was a big surprise. We were not expecting to see rates that strong," Sutton said.

Current models do not predict such large increases in carbon dioxide concentrations in that part of the ocean, which is one of the reasons those findings were unexpected. The data were collected from a set of seven buoys in the tropical Pacific, starting in 1998.

Because that part of the ocean is dominated by many natural cycles, including the El Niño Southern Oscillation and the Pacific Decadal Oscillation, the researchers had to work to disentangle the overall signal of carbon dioxide concentrations from the variability caused by natural changes.

"One of the reasons why we were really interested in putting these CO2 systems on the buoys was so that we could see the incredible natural variability that occurs in this region," Sutton said.

For the first few years of data collection, the researchers saw a clear signal of carbon dioxide growth rates changing as the ocean switched from El Niño to neutral to La Niña.

Shellfish and coral reefs at risk
Now that they have been collecting data for over 15 years, they have begun to see the effects of longer term cycles, such as that of a 20- to 30-year natural fluctuation called the Pacific Decadal Oscillation, Sutton said.

The scientists are also seeing the effects of climate change, one of which is an increasingly acidic ocean. The carbon dioxide they see in the tropical Pacific now is from water that was last in contact with the atmosphere 10 years ago, Sutton said.

In subtropical regions of the ocean north and south of the equator, the ocean gets into an equilibrium with whatever concentration of carbon dioxide is in the atmosphere at that time -- and every year, that atmospheric concentration has increased due to human emissions of CO2.

Then, the water is pushed deep into the ocean, where it collects more carbon dioxide due to natural biological processes. It eventually upwells back in the tropical Pacific.

"That water is carrying anthropogenic CO2 plus this natural CO2. So when it upwells along the equator, you are seeing both of those processes drive to produce CO2," Sutton said.

In parts of the U.S. coast, ocean acidification has led to negative impacts on the shellfish industry, since it affects the ability of those organisms to build their shells.

In the tropical Pacific, coral reefs have the potential to be affected by increasing concentrations of carbon dioxide and a more acidified ocean, since more acidic oceans have been shown to reduce the ability of reef-building corals to create their skeletons.

Scott Doney, a senior scientist and director of the Woods Hole Oceanographic Institution's Ocean and Climate Change Institute, who was not affiliated with the study, pointed out that "the research demonstrates the importance of continuous ocean observations (e.g., moorings, robots)."

"Only through careful analysis of long-term data sets can we separate the effects of natural climate variability from anthropogenic trends such as climate change and ocean acidification," Doney wrote in an email.

Watching acidification more closely.
As a follow-up to this research, Sutton and her colleagues plan to look at an even longer period of time using observations made not just from buoys but from measurements taken from ocean cruises as well.

The scientists hope to be able to determine more specifically what portion of the carbon dioxide concentration increase is due to human causes, and what portion is due to natural oscillations.

Yesterday, NOAA, the agency behind Sutton's research, also announced a new strategic plan guiding its monitoring and research on ocean acidification.

The plan goals include improving observation systems that monitor ocean acidification, such as the buoy systems used by Sutton and others.

Other goals include conducting laboratory and on-site research on acidification; developing comprehensive models to predict changes to the ocean's carbon cycle; assessing the cultural, subsistence and economic impacts of ocean acidification; and developing a data management plan for what is collected on the topic.

"Scientific study of ocean acidification is young enough that researchers are making surprising major discoveries every year," said Libby Jewett, director of NOAA's Ocean Acidification Program, said in a statement about the strategic plan.

"Federal investment in basic research, long-term monitoring, and multi-disciplinary, applied research, will allow U.S. scientists to develop the knowledge needed to inform policy and help prepare society for rapid shifts in ocean chemistry."

Scientific American is part of Springer Nature, which owns or has commercial relations with thousands of scientific publications (many of them can be found at www.springernature.com/us). Scientific American maintains a strict policy of editorial independence in reporting developments in science to our readers.